The customer recently had to remove and reinstall ASP.NET. After that Agents are not able to send basic inventory or request configuration and are failing speed tests against the NS.

Anything under Altiris/NS/Agent web directory throws a management server error when trying to load it.
Looking and the Agent logs we can see that the agents can't properly access these virtual directories under Altiris/NS/Agent web directory.

In the IIS logs the following entries were present as well:
2016-02-18 18:53:45 10.129.xx.xx HEAD /Altiris/NS/Agent/CreateResource.aspx nsversion=1 80 - 10.129.xx.xx - - 302 0 0 230
2016-02-18 18:53:45 10.129.xx.xx POST /Altiris/NS/Agent/CreateResource.aspx nsversion=1 80 - 10.129.xx.xx - - 302 0 0 10

2016-02-18 18:53:02 10.129.xx.xx HEAD /Altiris/NS/Agent/PostEvent.asp encrypted=1&priority=1&source=590F7B0D-3BC3-4B03-A3BB-FDB8C5F93A9A 80 - 10.129.xx.xx - - 302 0 0 29
2016-02-18 18:53:02 10.129.xx.xx HEAD /Altiris/NS/Agent/PostEvent.asp encrypted=1&priority=0&source=590F7B0D-3BC3-4B03-A3BB-FDB8C5F93A9A 80 - 10.129.x.xx - - 302 0 0 210

Looking at the Application Event logs, we can see that there is an assembly error regarding loading assembly  Altiris.web.Agent:

Event code: 3008
Event message: A configuration error has occurred.
Event time: 2/18/2016 10:39:35 AM
Event time (UTC): 2/18/2016 6:39:35 PM
Event ID: 676d9e3e2b334bdda3b8d7f3782cff19
Event sequence: 1
Event occurrence: 1
Event detail code: 0
 
Application information:
    Application domain: /LM/#####/1/ROOT/Altiris/NS/Agent-3246-131002943752941030
    Trust level: Full
    Application Virtual Path: /Altiris/NS/Agent
    Application Path: C:\Program Files\Altiris\Notification Server\AgentWeb\Agent\
 
Process information:
    Process ID: 3092
    Process name: w3wp.exe
    Account name: IIS APPPOOL\Symantec Agent AppPool
 
Exception information:
    Exception type: ConfigurationErrorsException
    Exception message: Could not load file or assembly 'Altiris.Web.NS.Agent, Version=7.6.1655.14, Culture=neutral, PublicKeyToken=d516cb311cfb6e4f' or one of its dependencies. The parameter is incorrect. (Exception from HRESULT: 0x80070057 (E_INVALIDARG))

Example of error messages seen on the Agent logs:

Operation 'Head' failed.
Protocol: HTTP
Path: /Altiris/NS/Agent/GetClientPolicies.aspx
Http status: 0
Id: {53B37FA4-31CE-4E1D-BD98-C9C6473D58CC}
Error type: HTTP error
Error result: 0x80042D24
Error code: 0
Error note: The remote web server is down or not responding properly, cannot determine SMP server version or SMP server version is not compatible.
Error message: Error 0x80042D24 (No description available)

-----------------------------------------------------------------------------------------------------

Date: 2/22/2016 10:33:40 AM, Tick Count: 428043437 (4.22:54:03.4370000), Size: 687 B
Process: AeXNSAgent.exe (12960), Thread ID: 9168, Module: AeXNetComms.dll
Priority: 1, Source: NetworkOperation

***************************************************************************************

Operation 'Head' failed.
Protocol: HTTP
Path: /Altiris/NS/Agent/PostEvent.asp
Http status: 0
Id: {168B0CAD-F8A9-409D-86DF-0594F7E8E8A2}
Error type: HTTP error
Error result: 0x80042D24
Error code: 0
Error note: The remote web server is down or not responding properly, cannot determine SMP server version or SMP server version is not compatible.
Error message: Error 0x80042D24 (No description available)

-----------------------------------------------------------------------------------------------------

Date: 2/22/2016 10:33:39 AM, Tick Count: 428042765 (4.22:54:02.7650000), Size: 679 B
Process: AeXNSAgent.exe (12960), Thread ID: 14504, Module: AeXNetComms.dll
Priority: 1, Source: NetworkOperation

Unknown. Based on Microsoft forums entries, one of the posts indicated that after installing and removing ASP.NET there is the chance of compilation corruption for some assemblies, and that usually resides under the "C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files" directory. Some of the files may not be compiled correctly.

In this particular instance the error was under the assembly "Altiris.Web.NS.Agent" (Altiris.Web.NS.Agent.dll) under "C:\Program Files\Altiris\Notification Server\AgentWeb\Agent\Bin" for the IIS APPPOOL\Symantec Agent AppPool.

To identify from where we are trying to load this assembly, there are different methods that Microsoft suggests. In this instance you can use Process Explorer.

The following steps were used to identify the proper location that we needed to clear out. We will use Altiris.Web.NS.Agent as example but you should be able to follow the same steps for others with similar errors:

1. Download Process Explorer
2. Find the w3wp.exe process associated to Symantec Agent AppPool and select it.
   
3. Go to Right-click>Properties
   
    
4. Go to the .NET Assemblies tab
   
   
   
5. Find the assembly for Altiris.Web.NS.Agent and take note of the assembly loading path
   

After identifying the proper path, in order to fix this compilation error, try the following steps:

1. Stop IIS (iisreset /stop).
2. Stop all Altiris Services.
3. Go to C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files and delete the "altiris_ns_agent" folder and its content.
4. Start Altiris Services and IIS (iisreset /start).
5. Request configuration and send basic inventory from the Agent UI and validate that the pages now are accessible

Note: In case that the issue still persists for the same assembly, you may want to check these other locations:

Depending on if your are running X64 you might need to clean up a couple more spots.

1. %TEMP%\Temporary ASP.NET Files
2. C:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files
3. C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files
4. C:\Windows\Microsoft.NET\Framework64\v2.0.50727\Temporary ASP.NET Files
5. C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files

This list will grow as if you have other versions of the framework installed.